LONDON — A senior British court Thursday dealt a severe blow to Prime Minister Theresa May’s plans to begin the process of exiting the European Union early next year, ruling she must get Parliament’s approval before she acts.

May’s lawyers had argued that she had the right to begin the Brexit process without first getting Parliament’s consent, given the referendum result. But a three-judge panel on the London-based High Court sided with a group of plaintiffs who contended that Parliament must first weigh in.

‘‘The most fundamental rule of the U.K.’s constitution is that Parliament is sovereign and can make and unmake any law it chooses,’’ the judges wrote. ‘‘As an aspect of the sovereignty of Parliament it has been established for hundreds of years that the Crown — i.e. the Government of the day — cannot by exercise of prerogative powers override legislation enacted by Parliament.’’

The court’s decision stunned British political and legal observers — just as the referendum outcome also defied predictions that voters would favor staying in the European Union. Until Thursday, most analysts believed the court would side with the government. The High Court in Northern Ireland had even ruled as recently as last week that May’s government could bypass Parliament.

Pro-Brexit advocates quickly denounced the decision, saying it amounted to a betrayal of the public’s will.

‘‘I now fear every attempt will be made to block or delay triggering Article 50,’’ tweeted Nigel Farage, a longtime Brexit champion. ‘‘They have no idea level of public anger they will provoke.’’

Suzanne Evans, a candidate to succeed Farage as leader of the UK Independence Party, added a condemnation of ‘‘activist judges’’ who ‘‘attempt to overturn our will.’’

‘‘Time we had the right to sack them,’’ she wrote..

The court ruling — assuming it is not overturned on appeal — sets up a crucial decision for the 650 representatives in Britain’s House of Commons. Members of the ruling Conservative Party were almost evenly split on whether Britain should stay in the European Union or leave when the country voted June 23. But solid majorities of the other major parties in Parliament — including Labour, the Scottish National Party, and the Liberal Democrats — all opposed an exit.

Most members of Parliament opposed Brexit in the lead-up to Britain’s June referendum, when voters opted for an exit by a 52-to-48 margin. But it risks setting off an angry backlash from voters who favor leaving the European Union and believe the issue was settled."

FROM YOUR OWN BBC, AND WONDERING HOW MANY OF YOU THERE IN THE U.K. LIKE THE REPORTING OF THE BBC:

"It is one of the most important constitutional court cases in generations. And the result creates a nightmare scenario for the government.

Theresa May had said she wanted to start Brexit talks before the end of March next year but this ruling has thrown the prime minister's timetable up in the air.

Campaigners who brought the case insist it was about "process not politics", but behind the doors of No 10 there will now be serious head-scratching about what the government's next steps should be.

This decision has huge implications, not just on the timing of Brexit but on the terms of Brexit. That's because it's given the initiative to those on the Remain side in the House of Commons who, it's now likely, will argue Article 50 can only be triggered when Parliament is ready and that could mean when they're happy with the terms of any future deal.

She rejected criticism that the case was subverting democracy and said it was "not about politics, this was about process".

"One of the big arguments [in the referendum] was parliamentary sovereignty... so you can't on the day you get back sovereignty decide you're going to sidestep or throw it away."

The government had argued it could use ancient prerogative powers to give effect "to the will of the people".

But the three judges looking at the case found there was no constitutional convention of the royal prerogative - powers used by ministers - being used in legislation relating to the EU.

They added that triggering Article 50 would fundamentally change UK people's rights - and that the government cannot change or do away with rights under UK law unless Parliament gives it authority to do so.

The UK voted by 52% to 48% to leave the European Union in a referendum on 23 June.

The EU's other 27 member states have said negotiations about the terms of the UK's exit - due to last two years - cannot begin until Article 50 has been invoked."

MY, MY MY.... LOOKS LIKE THE WILL OF THE PEOPLE, THE DECISION OF THE MAJORITY IS A THING OF THE PAST ALL OVER THE WORLD.

WE CAN POINT AT THIRD-WORLD NATIONS AND SAY, "LOOK AT THAT CORRUPT DICTATOR DOING WHAT PROFITS HIM AND DISREGARDING THE PEOPLE!", BUT WHAT DO WE HAVE IF NOT DICTATORS WHO OVERRIDE THE WILL OF THE PEOPLE?

WHAT HAVE OUR GOVERNMENTS BECOME, DEAR FRIENDS ACROSS THE LESSER POND, BUT CORRUPT DICTATORS?

BOTH OF THESE ARTICLES CLEARLY STATE THAT THE MAJORITY OF THE PEOPLE VOTED TO LEAVE THE EUROPEAN UNION, BUT THEY BOTH ALSO STATE THAT THE MAJORITY OF PARLIAMENT WANTS TO STAY IN THE EU.

HOW CAN THAT BE UNLESS YOUR PARLIAMENT, LIKE OUR CONGRESS, IS NOT VOTING THE WILL OF THE PEOPLE?

WE VOTING CITIZENS ARE SUPPOSED TO TELL OUR ELECTED ONES HOW TO VOTE...THEY ARE SUPPOSED TO HEAR OUR VOICES AND OBEY. THAT DOESN'T HAPPEN ANYMORE, HASN'T FOR A VERY LONG TIME....HAS IT?